Ninja makefile fallback

[kraenhansen]

This fixes #95 by adding a check and a fallback to Unix Makefiles.

Currently in draft because it building weak-node-api fails when the fallback is used:

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Latest commit: aba1cde

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[shirakaba]

Does the fact that Chromium is dropping Ninja in favour of Siso have any impact on this?

[kraenhansen]

Does the fact that Chromium is dropping Ninja in favour of Siso have any impact on this?
Likely not.

Using Ninja by default is just to get the fastest builds possible and CMake doesn't support Siso:
https://cmake.org/cmake/help/latest/manual/cmake-generators.7.html#command-line-build-tool-generators

This suggested change is just a simple way to degrade gracefully, in case the Ninja is not installed, to not force library authors into using Ninja - if they absolutely doesn't want to install it.